1. Field of Invention
The present invention relates to demodulation of quaternary phase shift keyed (QPSK) data.
2. Description of Prior Art
Prior art communication systems have used balanced quaternary phase shift keying (QPSK) of two data bit streams onto a carrier.
Prior art QPSK demodulators, such as in U.S. Pat. No. 3,594,651, multiplied the incoming QPSK signal with a local QPSK reference signal generated by the summation of two PSK signals and formed error signals from the product of such multiplication to control the output of a voltage controlled oscillator in a phase-locked loop. These amplitude adjusting demodulators involving a summation of two signals were susceptible to gain and bias fluctuations and other factors, such as temperature, affecting gain stability or signal amplitudes.
However, where two data streams are at different data rates, it has recently been found desirable to unbalance the amount of phase shift indicating data bits, for example using 26.degree., 206.degree., -26.degree. and -206.degree., to equalize bit error rate probability in the data streams. This technique is particularly of interest where the data rates are significantly different, such as in spacecraft communication. In unbalanced quadriphase modulation, the carrier phase shifts between successive samples are not multiples, such as 90.degree., 180.degree., 270.degree., etc., of a common phase 90.degree. increment as was the case with balanced quadriphase, since signal power levels in the data streams differ in order to equalize bit error rate probability.
To account for the unbalance in the signal set, past quadriphase demodulators were modified to produce an error signal by summing the tracking signals in different voltage ratios defined by the power ratio between the two data channels. However, this voltage magnitude adjustment still led to sensitivity to internal amplitude changes.